Various methods have been invented for forming diamond from vapor phase, such as a hot-filament CVD method, a microwave plasma assisted CVD method and so forth. The microwave plasma assisted CVD method is especially suitable, among others, for forming a high-purity polycrystalline diamond film and an epitaxial diamond film, whereby a high-quality diamond film can easily be obtained compared to the case with other methods. The other methods are associated with some problems that degrade the quality of the diamond film. For example, the hot-filament CVD method involves metal contamination from filament, and a plasma jet method involves metal contamination from an electrode. Moreover, in a combustion flame method, nitrogen in the air is mixed into diamond, degrading the quality of the diamond film. Thus, the microwave plasma assisted CVD method has been widely used as a method of obtaining a high-quality diamond film, and recently, developments have been propelled for obtaining a large area of high-quality diamond film.
The microwave plasma assisted CVD method has an advantage in that such a high-quality diamond film can easily be obtained, but suffers a drawback in that the resulting film is varied in its thickness and quality in a wide range of distribution, especially when compared to the case with the hot-filament CVD method. Thus, it is particularly difficult to obtain a large size of diamond film having a uniform thickness and quality by the microwave plasma assisted CVD method. Currently, there is not even a guideline for adjusting the variation as described above, and such a guideline is still being searched for. As a guideline in forming a diamond film by the microwave plasma method, it is known to use a temperature of a substrate measured using a radiation thermometer and a thermocouple within a reactor, and further spectrum analysis by plasma emission spectroscopy or the like. However, the substrate temperature measured by the radiation thermometer is essentially associated with the plasma emission, making it difficult to obtain an accurate temperature of the substrate. Furthermore, when the thermocouple is used for a temperature measurement, the temperature cannot directly be obtained unless the substrate is in direct contact with the thermocouple. Even if such direct contact was possible, such contact would cause disturbance, which affects formation of the diamond film. Whereas, when the plasma emission spectroscopy is used for diagnosing a plasma state, observation on the spot is possible without any contact, causing no disturbance to the plasma state. Thus, conventionally, the diagnosis using the plasma emission spectroscopy has been actively performed. The measurement using the plasma emission spectroscopy has been successful in certain ways, for instance, contamination by nitrogen, which significantly interferes with the formation of the diamond film, can be found instantly. However, the plasma emission spectroscopy has not yet reached the level where the quality and the deposition rate of the diamond film can be predicted.